Author HEY! from the blog you're welcome has written a really touching and strikingly readable post on why she's fat positive. It's practically a mini-manifesta. Here's an excerpt:

"When I tell people I’m fat, the most common reaction is panic: they assume I’m calling myself ugly, and they feel pressured or compelled to disabuse me of that notion. But people clearly notice my size all the time. Servers at restaurants notice it when they seat me. People on the bus notice it when they decide when and whether to make an empty seat available for me. Nurses notice it when they tell me I’ll have to be weighed as part of my physical, and then wince. It’s the worst-kept secret around. So I just put it out there: I’m fat

Telling people I’m fat makes them uncomfortable. But telling them I’m fat positive makes people upset. Some get angry (“you’re endangering your health, and if you’re talking about this, you’re endangering other people’s health, too”). Some get shaming (“oh, so that’s why you dress that way. I just figured you didn’t know they were called skinny jeans”). Some even start to mourn (“I’m just worried about what will happen to you if you don’t even try.”). Most are just perplexed and they shut down, wondering on a very basic level why I think fat is okay.

Remarkably, very few people ask. So I figured I’d just say it.

I’m fat positive because I’m a feminist, and I refuse to acknowledge in the magical thinking that if you’re small enough, quiet enough, compliant enough and saccharine enough, you will somehow be enough.

I’m fat positive because I can’t afford to pay for two airline tickets just because the airline industry has decided that my body is the problem—not their outdated seats that haven’t changed in decades.

I’m fat positive because I’ve been fat my whole life. No matter how much I work out or how little I eat, my clothing has never dropped below a size 20 (I know!). I could spend my life in a gym, chasing some mirage that my body will never be, or I could focus on eating and moving in a way that makes me healthy and happy. Or, even more radically, I could not think about dieting, and know that my health is my own damn business. Either way, on the weight continuum, I’ll be somewhere between “superfat” and “ginormous.”

I’m fat positive because every day, fat people give up on all kinds of priorities and dreams because they’re fat. Granted, that’s a piss-poor reason to give up, but you know what? Social messages reinforce that thinking every day. I can’t go to the gym: I’m fat & I’ll be humiliated. I can’t date anyone: who would want to date a fatty? I can’t wear that outfit: I’m fat. Hell, I gave up on acting in college because I didn’t think I stood a chance. I may not have been a great actor, but I don’t know because I knew that being fat was rarely a leg up in auditions.

I’m fat positive because of the pervasive myth that fat women must be lesbians—the underlying assumption being that queer women can “give up” on their bodies, because they don’t “need” to attract men. I’m fat positive because I’m queer, and that shit is homophobic AND sexist."